Photo provided by the anti-government activist group Syrian Observatory for Human Rights, which has been authenticated based on its contents and other AP reporting, shows Jihadi fighters from the al-Qaida-linked Nusra Front and other rebel factions, rest after clashes with Syrian troops in Wadi Deif in the northwestern province of Idlib, Syria, Dec. 14, 2014. AP

Photo provided by the anti-government activist group Syrian Observatory for Human Rights, which has been authenticated based on its contents and other AP reporting, shows Jihadi fighters from the al-Qaida-linked Nusra Front and other rebel factions, rest after clashes with Syrian troops in Wadi Deif in the northwestern province of Idlib, Syria, Dec. 14, 2014. AP

The Assad regime sustained a major setback in northern Syria this week with the loss of two major military bases, and the big winner in the battle was al Qaida's Syrian affiliate, the Nusra Front, which led the assault and captured what’s thought to be an enormous arsenal.

The victory by Nusra, which has been on the U.S. list of terrorist groups since 2012, marked another loss of American influence on the battlefield. It came a month after Nusra ousted two U.S.-supported moderate rebel groups from their bases in Idlib province.

But the main loser was the Assad regime. Of the estimated 1,000 or more regime troops stationed at the Wadi al Deif and al Hamidiyah bases, only about 500 had reached the city of Hama – to which the troops withdrew – as of Tuesday night, about half of them wounded. A high source in the moderate Free Syrian Army estimated that 350 regime troops had died in the fighting, and rebel commanders claim to have captured more than 200.

Official Syrian news media made no mention of the rout, which will impair the regime’s ability to supply its forces in Aleppo, the country’s biggest city, and in Idlib, a major center in northwest Syria. The setback is bound to affect support for President Bashar Assad among the Alawite minority, his core constituency, from which a high proportion of military conscripts are drawn.

U.S.-backed rebels, whose salaries Washington suspended earlier this month, played a secondary role, damaging several regime tanks as they were withdrawing from Idlib. Even more embarrassing for the U.S. was that Nusra fighters used American-supplied TOW missiles they’d captured last month when they ousted the U.S.-backed Syrian Revolutionaries Front and the Hazm movement from the Azawiya mountains in Idlib province.

Media activists equipped with cameras documented repeated instances, starting over the weekend, in which the Nusra Front hit regime tanks using American TOW missiles.

Taking part in the frontal attack were Nusra, Jund al Aqsa – another extremist group – and Ahrar al Sham, a nationalist Islamist force that, like Nusra, has been the target of American air bombardment.

In a video recorded Monday, an Ahrar al Sham spokesman said the fighting began last Thursday with an assault on 40 military targets. The forces attacked the two bases Saturday, and by Sunday regime troops began to flee, heading first to Mar Hatat and Bseda, two villages south of Idlib. Under continued assault, they escaped into open desert east of the main north-south highway that links Aleppo with Hama, where many were killed or captured.

What was left of the government force finally reached Hama early Tuesday – some 500 fighters, together with seven tanks and six armored vehicles. Sources in Hama, which is controlled by the regime, told McClatchy that government hospitals stopped treating civilians and were receiving only wounded soldiers.

Free Syrian Army officers told McClatchy that at least two of their units, Fursan al Haq and Division 13, went on the attack as regime forces were in retreat, using U.S.-supplied TOW missiles. They said their fighters had helped turn the retreat into a rout for the regime, possibly doubling the casualties.

Rebels from secular and Islamist groups had been trying for three years to seize the huge Wadi al Deif base, but with little success. They began a siege of the base at the beginning of this year.

The rebels were able to stop the regime at the town of Morak, north of Hama, disabling more than 100 tanks using TOW missiles and seizing many towns, especially Khan Sheikhun, but amid complaints by Free Syrian Army forces of ever-diminishing U.S. military supplies, the regime recaptured Morak.

The attack on the bases came a month after the Nusra Front had launched a campaign against the Syrian Revolutionaries Front, a Western-backed secular group in Idlib, and eliminated a moderate force of about 7,000 fighters, capturing most of their weapons, including tanks and anti-armored-vehicle rockets.

There’s little question that the Islamists’ gains constituted a setback for the United States.

“This is a loss to the United States, which says that it supports moderate rebels, while Nusra Front, Jund al Aqsa and Ahrar al Sham are controlling most of Idlib now,” said Rami Abdulrahman, who runs the Syrian Observatory for Human Rights, an independent group in Britain that monitors the war in Syria.

Col. Ahmad Saud, the leader of Division 13, a Free Syrian Army umbrella group that’s received U.S. support, said the latest rebel gains had enormous strategic importance, in particular for Marat al Numan, which is close to the two bases and astride the main north-south highway. Rebels recaptured it several months ago, but they were under constant threat from regime forces at the bases.

“Now we have a vast area of land extending from Aleppo to Hama province which is open,” Saud said in a Skype call from Marat al Numan.

Saud acknowledged that Nusra had used TOW missiles, saying the terrorist group had gotten them from the Syrian Revolutionaries Front and the Hazm movement in the fight against those two groups last month.

Even more embarrassing was that a defector from Hazm appeared in the video released by Nusra, launching a TOW missile. Saud said the fighter, who uses the nom de guerre Abu Omar, had been forced to work with Nusra. He also said Nusra had now exhausted its supply of captured TOW missiles: “Nusra now has no more TOWs, and the launcher it still preserves will become useless.”

Basel al Darwish, an opposition activist close to the scene of the battle, said the new victory would pave the way for thousands of internally displaced people to return to their towns and villages around the bases.

“The people of 60 towns and villages can come back to their homes now. This applies to the city of Marat al Numan, whose population is more than 200,000.”

But the Syrian air force may prevent civilians from savoring the fruits of victory. Activists said regime airplanes had raided Marat al Numan on Tuesday, killing 10 civilians. The Local Coordinating Committees, a network of local reporters, said 13 people had been killed in a regime raid on a hospital in the town of Kefrenbil, to the west of Marat al Numan.